Both have their drawbacks. You can't Suspension Field a smaller dude which could be something you need to do in some games. It also gets blown up by enchantment removal which will likely come in postboard from the decks that you really want Suspension Field against. Typically if you are behind against the decks you'd want Suspension Field for, it being a better topdeck than Ride Down is likely irrelevant because you are not likely to win from behind with this deck anyway.
Sometimes they just have to block or you win....It fits the general purpose of the deck better. If you're behind you're behind and probably going to lose anyway. Can always side em out for deflecting palms or something second game when they know you run em. *in reference to ride down*